1. Technical Field
The present invention relates to a sensor panel and an image detecting device, and in particular, to a sensor panel provided with an electrostatic protective circuit that protects a circuit due to current flowing through when overvoltage is generated, and to an image detecting device using this sensor panel.
2. Related Art
Radiation image detecting devices, such as FPDs (flat panel detectors) and the like in which an X-ray sensitive layer is disposed on a TFT (thin film transistor) active matrix substrate and which can convert X-ray information directly into digital data, have been put into practice in recent years. As compared with a conventional imaging plate, an FPD has the advantages that an image can be confirmed immediately and that video images as well can be confirmed, and the popularization of FPDs has advanced rapidly.
In this type of radiation image detecting device, it is important to detect an X-ray image at a size that projects a human body, and therefore, substrates of large sizes exceeding 30×30 cm are needed. However, it is difficult to manufacture a substrate of such a large size from a silicon substrate. Therefore, currently, TFT active matrix substrates that are formed on a thin plate glass are mainly being used.
TFT active matrix substrates are employed as driving substrates for LCDs (liquid crystal displays), and are stable technologically and in terms of cost as well. Therefore, TFT array substrates for image detecting devices as well are mainly manufactured on assembly lines for TFTs for LCDs from the standpoint of cost.
A circuit diagram of a conventional TFT active matrix substrate 10′ for an image detecting device is shown in FIG. 11.
As shown in FIG. 11, the TFT active matrix substrate 10′ is structured such that a large number of pixels are arrayed in a two-dimensional form. The pixel is structured to include a charge collecting electrode 11′ that collects charges generated at an image sensor portion (not illustrated), a charge storage capacitor 5′ storing detected charge signals, and a thin film transistor (hereinafter called “TFT switch”) 4′ for reading-out the charges stored in the charge storage capacitor 5′. Further, plural scan lines 101′ for turning the TFT switches 4′ on and off, and plural data lines 3′ for taking-out the charges stored in the charge storage capacitors 5′, are provided at the TFT active matrix substrate 10′. One electrode of each of the charge storage capacitors 5′ is grounded via a line (not illustrated) and is made to be ground level. Note that, in FIG. 11, the one electrode of the charge storage capacitor 5′ is illustrated as being connected to ground.
The respective data lines 3′ and the respective scan lines 101′ of the TFT active matrix substrate 10′ are connected to a common line 110′ via bidirectional diodes 30′ for circuit protection respectively, in order to prevent electrostatic breakage at the time of manufacturing.
An example of the structure of one diode 31′ that structures this conventional bidirectional diode 30′ is shown in FIG. 12.
In a TFT active matrix substrate that uses an amorphous silicon TFT, the diode 31′ can be structured easily by connecting the gate electrode and the drain electrode of the TFT switch, as shown in FIG. 12.
FIG. 13 shows an equivalent circuit focusing on one TFT element of the TFT active matrix substrate 10′ shown in FIG. 12.
As shown in FIG. 13, at each TFT element of the TFT active matrix substrate 10′, between the gate electrode and the data electrode of the TFT switch 4′ is equivalent to being connected by the bidirectional diode 30′ that is structured by the anodes and cathodes of two of the diodes 31′ respectively being connected to one another in parallel. Therefore, if the electrode potential of one becomes high, charges flow to the other, and the potential can be prevented from becoming high.
In the bidirectional diode 30′, it is important that, at the time of manufacturing, sufficient current flow (the bidirectional diode 30′ be low resistance) in order to protect the TFT active matrix substrate 10′, whereas on the other hand, at the time of image detection, in order to suppress noise and artifacts, it is important to not to flow the leak current. For this purpose, as shown in FIG. 11, it is usually the case that the bidirectional diodes 30′ are inserted between the common line 110′, and the respective data lines 3′ and the respective scan lines 101′. Because the resistance value of the bidirectional diode 30′ varies greatly due to the potential difference between the two terminals, the aforementioned characteristic can be obtained.
However, in a case in which the bidirectional diode 30′ of such a structure is applied to a radiation image detecting device, the charges generated by the irradiation of X-rays and the like are stored between the photoelectric conversion layer and the interlayer insulating film. It is clear that, due thereto, there are cases in which the TFT element structuring the bidirectional diode 30′ malfunctions.
This phenomenon will be described in further detail hereinafter.
An example of the layer structures of a pixel portion, that detects an image, and the bidirectional diode 30′ portion of the TFT active matrix substrate 10′ is shown in FIG. 14.
In a direct-conversion-type radiation image detecting device in which a semiconductor film 6′ of Se or the like is formed as a photoelectric conversion layer on the TFT active matrix substrate 10′, a bias electrode 7′ is provided at the top layer of the semiconductor film 6′. By applying high voltage to the semiconductor film 6′, the semiconductor film 6′ works as a photoelectric converting element. For example, in the case of the semiconductor film 6′ which is mainly Se, high voltage of around 1 to 10 kV is applied to the Se film whose film thickness is about 0.1 to 1 mm.
On the other hand, also above the bidirectional diode 30′ at which the charge collecting electrode 11′ does not exist, almost all of the voltage that is applied to the bias electrode 7′ is applied to the semiconductor film 6′. For example, in a case in which the Se is 1 mm (relative permittivity of 7), an interlayer insulating film 12′ is 2 mm (relative permittivity of 3.5) and the voltage applied to the bias electrode 7′ is 10 kV, greater than or equal to 990 V is applied to the Se layer.
In this state, when X-rays are irradiated, charges are generated also at the semiconductor film 6′ above the bidirectional diode 30′, in the same way as at the pixel portion. However, because there is no flow-out destination for charges above the bidirectional diode 30′, the generated charges are stored and are held at the defect level within the semiconductor film 6′, or at the interface between the semiconductor film 6′ and the interlayer insulating film 12′. Due thereto, the potential becomes high at the upper layer portion of the bidirectional diode 30′, and voltage is applied via the interlayer insulating film 12′ to the channel portion of the TFT element structuring the bidirectional diode 30′. In this way, at the TFT element, operation that is similar to a case of applying bias to the back gate is brought about due to the defect level of the interface, and the resistance value decreases.
Originally, high resistance must be maintained without current flowing at the bidirectional diode 30′, except for cases in which high voltage is applied to the signal line for protection at the time of image detection.
However, when the charge amount stored at the interlayer insulating film 12′ becomes large, the resistance value of the bidirectional diode 30′ becomes low regardless of the voltage of the line. Leak current is generated via the bidirectional diode 30′, and becomes a cause of the occurrence of artifacts and an increase in noise.
Note that, as related art, Japanese Patent Application Laid-Open (JP-A) No. 11-68078 discloses a structure of covering a driving means by shield electrodes in order to prevent electrostatic breakage.
However, the technique of JP-A No. 11-68078 restricted to the covering of a driving means by shield electrodes, and cannot prevent the occurrence of malfunctioning of an electrostatic protective circuit. Therefore, there is the problem that generation of artifacts and an increase in noise cannot be suppressed.